716 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



have an age of 1,852,000 years, as indicated by the study of radioactive distinte- 

 gration products. 



Investigations of the rocks along the St. Lawrence Kiver Valley in 1843 by 

 Sir William Logan, the first director of the Geological Survey of Canada, re- 

 sulted in the subdivision and differentiation of the pre-Cambrian and the group- 

 ing of the granites and gneisses under the term Laurentian. Later his studies 

 were extended into the area north of Lake Huron where he found a series of 

 slates, quartzites, and conglomerates containing pebbles derived from the under- 

 lying Laurentian granites. He called these rocks Huronian, from their occur- 

 ence on the northeast side of the lake. He recognized a third series of still 

 younger volcanic rocks containing copper and interbeded sedimentary rocks 

 which he considered as a part of the Huronian. Later, in 1876, these rocks were 

 named Keweenawan by Brooks. These studies were followed by detailed inves- 

 tigations of particular areas by Dawson, Bell, Coleman, Collins, and Barlow 

 in Canada and by Van Hise, Leith, Irving, Lawson, Petti John, and many others 

 in the United States. The areas involved included Newfoundland, Nova Scotia, 

 New England, parts of the Appalachian region, Montana, the Grand Canyon 

 area in Arizona, the Llano area of Texas, and many parts of the Cordilleran 

 region. 



The term Archaean system was defined by Dana in 1872 so as to apply to all 

 pre-Cambrian rocks. Extensive outcrops of greenstones and green schists in 

 the Lake of the Woods area in Canada, which overlie the younger intrusive Lau- 

 rentian granites earlier described by Logan, were studied by A. C. Lawson in 

 1885 and named the Keewatin series. Lawson also defined the Coutchiching 

 series, consisting of mica schists that were originally sediments, well exposed in 

 the Rainy Lake area and believed that these rocks underlay the Keewatin. He 

 later observed that a thick accumulation of slaty shales, with a basal conglom- 

 erate consisting of boulders derived from the Laurentian granites, rested upon 

 the old intrusive rocks and that these in turn were invaded by a later granite. 

 Lawson named these sedimentary rocks the Seine River series and applied the 

 term "Algoman granite" to the later intrusives. Other rocks of similar age in 

 another region were named the Timiskaming series. Thus there were recognized 

 two periods of batholithic intrusion prior to the deposition of the Huronian sys- 

 tem. Collins, in 1922, determined that a third intrusive interval occurred after 

 the accumulation of the Keweenawan volcanics and was accompanied by strong 

 mountain-making movements which brought pre-Cambrian time to a close. 



The downwarping of an extensive peneplain carved out of the Algoman 

 Mountains formed an area for the deposition of the Huronian sediments which 

 have been defined as the Bruce, Cobalt, and Animikie formations. North of Lake 

 Huron the lower beds consist of nearly twenty thousand feet of coarse sand- 

 stones and conglomerates containing striated boulders, which were interpreted 

 by A. P. Coleman in 1908 as a tillite indicative of an early ice age. The Ani- 

 mikie series was named in 1873 by T. S. Hunt, of the Canadian Geological 

 Survey, and consists of metamorphosed and unmetamorphosed rocks, including 

 slates, quartzites, graj-vvackes, schists, and eruptive rocks. Originally these were 

 thought to be a part of the Keweenawan series but later this series was found 

 to be unconformable on the Animikie. Because of the great differences of opinion 

 concerning the classification of these rocks a committee of geologists was ap- 



